Lithium-based battery systems having carbon black or manganese dioxide cathodes, and whose electrolytes have the general formula R.sub.p O.sub.r X.sub.t, where R may be sulfur, carbon, or silicon; O is oxygen; and X may be fluorine, chlorine, bromine, or iodine, and p&gt;0, r&gt;0, and t&gt;0, suffer from the problem that, after storage, the buildup of a passivating film on the anode results in reduced voltage under load and a time delay before the battery reaches steady-state output voltage.
One approach for overcoming this problem is the addition of various chemicals to the electrolyte. Another approach is the application of special coatings to the anode. These procedures are time-consuming during manufacture and costly.
Yet another approach is to modify the materials used in making the cathode and anode by washing them with acetone or methanol, as described in U.S. Pat. Nos. 4,526,881 and 4,543,305. This procedure removes impurities from the carbon black from which the cathode is made. Such impurities, if not removed, migrate to the anode, forming a passivating film on the anode and causing a low output voltage and a delay in reaching a steady-state voltage. The drawback of this approach is the fire hazard inherent in the washing with acetone and methanol, and the potential exposure of personnel to toxic concentrations of acetone or methanol vapor.
These disadvantages are overcome by a method, disclosed in the inventors' U.S. Pat. No. 5,328,782, whereby the cathode material is exposed to a low-pressure, room-temperature gas plasma. The method is for an electrical cell having a lithium anode, a carbon black cathode, and sulfur dioxide, acetonitrile, and lithium bromide as electrolyte.
SUMMARY OF THE INVENTION
It has now been discovered that the carbon black cathode material, either in powder form or formed into a cathode, may be treated with a gas plasma in accordance with this invention, and used in making an electrical cell having a conductive electrolyte consisting of acetonitrile, lithium bromide, and a compound of the general formula R.sub.p O.sub.r X.sub.t, where R may be sulfur, carbon, or silicon; O is oxygen; X may be fluorine, chlorine, bromine, or iodine; and p&gt;0, r&gt;0, and t&gt;0. The electrolyte may also consist of any other water-soluble salt in conjunction with a compound of the general formula R.sub.p O.sub.r X.sub.t. The result of this treatment is an increase in cell voltage and a decrease of the time required for the cell to reach a steady-state voltage. It is believed that the gas plasma treatment method of this invention removes the impurities initially contained in the carbon black cathode material, thereby preventing such impurities from migrating to the anode of the electrical cell, forming a passivating film thereon, and causing a low output voltage and a long time delay in reaching a steady-state output voltage.